Recently, a large number of electric devices have been used in homes or companies, and along with this, a problem of adverse influence of electro-magnetic interference (EMI) noise on other electronic devices arises. Such EMI noise is largely classified into two types. One is conducted interference transmitted through a power line and another is radiated interference directly radiated from devices. As one of methods for evaluating the conducted interference of them, a noise terminal voltage test is given. The test is for measuring high-frequency noise signal voltage induced in a power voltage terminal of an electric device.
Various countries have established strict standards on the noise terminal voltage. For example, there are standards such as CISPR (International Special Committee on Radio Interference) as an international standard, FCC (Federal Communications Commission) in the United States, and VCCI (Voluntary Control Council for Interference by Information Technology) in Japan. For example, CISPR22 defines a strict standard value for a wide frequency band of 150 kHz to 30 MHz. In accordance with this, it has been performed that a measuring device for the noise terminal voltage as shown in FIG. 18 has been set in a radio wave anechoic chamber to measure conformity to standards.
FIG. 18 shows a noise-terminal-voltage measuring system used for measurement of conformity to standards. In the system, power voltage from commercial power is supplied to a simulated power circuit network 101C in a measuring device 101 via a power cable 100C (here, a pair of power lines and a ground line are shown by a single thick-line), and in turn supplied to a device to be measured 102 via power lines 101A, 101B and a ground line 101G. Noise generated by the device to be measured 102 is measured by a spectrum analyzer 103. The simulated power circuit network 101C is inserted between the device to be measured 102 and a power source. The network 101C is for supplying power while keeping impedance seen from a power terminal of the device to be measured 102 to a defined value (50 to 150 Ω), and isolating a measuring circuit from external noise at a power source side, which is a signal detector indispensable for accurately detecting a noise signal generated in the device to be measured 102.
The measuring device 101 has a switch 101S, and can selectively measure noise at a power line 101A side or noise at a power line 101B side by changing the switch 101S.
FIG. 19 shows an example of a specific circuit of the measuring device 101. The circuit is described, for example, in a connection diagram of the simulated power circuit network KNW-242C manufactured by KYORITSU CORP.
The measuring device 101 has a power input terminal J1, power output terminal J2, and signal output terminal J3. A simulated power circuit network 101C is provided on power lines 101A, 101B between the power input terminal J1 and the power output terminal J2. The simulated power circuit network 101C has inductance elements L1 and L3 connected in series inserted into the power line 101A, and inductance elements L2 and L4 connected in series inserted into the power line 101B.
The inductance element L1 is connected to ground at a side of the power input terminal J1 via a resistor R1 and via a capacitor C1 and a resistor R3 connected in series. A connection point between the inductance elements L1 and L3 is connected to ground via a capacitor C3 and a resistor R5 connected in series, and the inductance element L3 is connected to ground at a side of the power output terminal J2 via a capacitor C5 and a resistor R7 connected in series.
The inductance element L2 is connected to ground at a side of the power input terminal J1 via a resistor R2 and via a capacitor C2 and a resistor R4 connected in series. A connection point between the inductance elements L2 and L4 is connected to ground via a capacitor C4 and a resistor R6 connected in series, and the inductance element L4 is connected to ground at a side of the power output terminal J2 via a capacitor C6 and a resistor R8 connected in series.
A connection point P1 between the capacitor C5 and the resistor R7 and a connection point P2 between the capacitor C6 and the resistor R8 are connected to the switch 101S, and one of noise signals from the connection points P1, P2 appears at a signal output terminal J3, and the other is connected to ground in response to change of the switch 101S.
In the simulated power circuit network 101C, when the power line 101A is noted, an LC filter including the inductance elements L1, L3 and the capacitors C1, C3 is configured, and when the power line 101B is noted, an LC filter including the inductance elements L2, L4 and the capacitors C2, C4 is configured. The LC filters are configured in a manner that they exhibit high impedance to both noise signals from the power input terminal J1 and from the power output terminal J2, thereby while they transmit low-frequency AC voltage, the power input terminal J1 is isolated from the power output terminal J2 with respect to a high-frequency noise signal.
An air-core coil, which is configured without inserting a core into a coil, is used for the inductance elements L1, L3 and the inductance elements L2, L4 in order to make a frequency characteristic to be flat to a high frequency band (that is, in order to enable signal separation independently of frequencies). This is because if the coil has the core, a signal separation characteristic has frequency dependence.
From the consideration of recent social environment about noise, the following points are given.    1) Energy saving by a top runner method is promoted.    2) Harmonic distortion in a power line becomes problematic; therefore a countermeasure circuit for harmonics is generally mounted.    3) Power for some home electric appliances including plasma display tend to be significantly increased.    4) In addition to information devices, home electric appliances are generally controlled by a microprocessor.    5) In addition to a noise problem due to rotational electric devices such as an electric tool, a problem of switching noise is actualized in a lighting fixture, an air conditioner and the like because of introduction of inverter control.
In this way, recently noise generated by the electric devices tends to increase particularly due to increase in switching control of a power source of a device, increase in number of primary phase control circuit, and furthermore multiplexing of a switching circuit. Therefore, to examine whether the noise meets the standard, necessity of measurement of the noise terminal voltage using the measuring system as shown in FIG. 18 is increased larger and larger.
Such noise terminal voltage includes a common mode (noise) signal and a normal mode (noise) signal. The common mode signal is transmitted over two conductive lines (for example, a pair of power lines) in the same phase, and the normal mode signal is transmitted by two conductive lines and generates potential difference between the two conductive lines. In many cases, amplitude or a frequency band of each mode of the signals varies depending on a circuit configuration of an electric device as a measuring object.
However, while the measuring device as shown in FIG. 18 takes the noise terminal voltage as the measuring object, it measures the common mode signal and the normal mode signal together (in a mixed manner). Therefore, signals may not be separated to be measured for each mode, and therefore the amount of information is insufficient for detail and accurate cause analysis of noise, consequently countermeasure for noise need to be performed with trial and error. Therefore, there has been a drawback that long experience and much time and effort are necessary for the countermeasure for noise; consequently development cost is increased in developing a new electric product.